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When do International Space Station crew members lose bone and muscle mass during their six-month missions? Why do they return deficient in Vitamin D? These are just some of the questions that may be answered by Nutrition -- NASA's most comprehensive in-flight study of the internal workings of the human body during long-duration spaceflight.

The Nutritional Status Assessment, or "Nutrition" experiment, is about much more than eating the proper foods on orbit. This experiment measures physiological indicators of the changes in the human body during spaceflight. Blood and urine samples will be collected and analyzed to understand a wide variety of systems, including markers of bone metabolism -- processes which produce, maintain and destroy bone; oxidative damage -- the effects of radiation; vitamin and mineral status; and hormonal changes and how they relate to stress, bone and muscle metabolism.

The findings are expected to give researchers a better understanding of what physiologically happens to crew members in space and when it happens during a mission.

The study will monitor 12 astronauts in missions over a three-year period. Results are expected to help NASA define nutritional requirements for astronauts and to develop food systems to further the Vision for Space Exploration on future missions to the moon and Mars. The findings also are likely to help better understand the impact of countermeasures -- such as exercise or pharmaceuticals -- to evaluate their effectiveness and to help refine protocols and requirements.

All U.S. astronauts on the space station already undergo a Clinical Nutritional Assessment profile, with blood and urine samples collected before and after their mission. Nutrition expands this procedure by collecting samples during the mission, along with one final set of samples 30 days after an astronaut returns to Earth. Additional tests have also been added to the existing protocol, to provide a more comprehensive evaluation on crew health and nutritional status.

"Blood and urine samples give us a window into how the body is reacting during flight, and could provide us with evidence of the nature of spaceflight-induced bone loss and how it progresses over time," said Dr. Scott M. Smith, principal investigator for the experiment at NASA's Johnson Space Center in Houston. "If we can pinpoint exactly when the losses and other physiological changes begin to occur and if they slow over time, we may be able to adjust our countermeasures accordingly. Furthermore, with pre- and postflight studies, you don't have the ability to understand the effects of mid-mission events, like spacewalks, on nutritional and health status. The blood and urine samples collected here will capture that information for us."

The inflight experiment collections began in October during the six-month-long Expedition 14 -- the 14th research mission to the space station that arrived there in September. Commander Michael Lopez-Alegria, who also serves as a NASA science officer, is periodically collecting blood and 24-hour urine samples and storing them in a cold storage unit that maintains experiment samples at a temperature of -80 degrees Celsius. The unit is called the Minus-Eighty Laboratory Freezer for ISS, or MELFI. In these super-cold temperatures, the samples await their return to Earth for analysis.

The Human Research Facility-2 is a space station rack that houses medical experiments that could increase the overall understanding of the human body during long-duration missions. The rack's refrigerated centrifuge -- a device that separates biological substances of varying densities by spinning them at a high rate of speed -- is being used for this experiment to separate the serum component from the blood cells before they are placed in the freezer.

"Knowledge gained from this experiment will help us target countermeasures to the harmful health effects of spending six months floating in microgravity," said Dr. Julie Robinson, Acting Program Scientist for the International Space Station at NASA's Johnson Space Center in Houston. "The importance of the investigation extends beyond the space station to keeping crews healthy on exploration-class missions."